1. Field of the Invention
This invention relates to planar optical diffraction elements for use as lenses or the like, and to a method of making the elements employing thick holographic techniques.
2. Prior Art
Planar optical elements incorporating diffraction patterns are useful as lenses, for performing Fourier transformation and spatial filtering in coherent optical processing apparatus, imaging and various like purposes. They are commonly formed by holographic methods involving the coherent recordation (on a photographic media) of the interference pattern between a wavefront emanating from a light source, and a coherently generated reference wavefront.
In the past, two main techniques have been available for forming these holographic optical elements (HOE). One method employed object and reference wavefronts arrayed on-axis relative to the optical element to produce a thin two-dimensional fringe pattern in the plane of the optical element and the other used a reference beam off-axis relative to the object beam to produce a holographic record through the thickness of the photographic media. The on-axis HOE provides a low dispersion and minimal optical aberrations when compared to the off-axis thick HOE's. On the other hand the thick HOE's provide a relatively high ratio of diffracted to undiffracted light. In the past, either a thick or thin HOE has been chosen, depending upon the nature of its intended use.
The present invention is broadly directed toward a unique form of diffraction optical element which combines the most advantageous characteristics of conventional thin and thick HOE's and provides a better ratio of diffracted light energy to undiffracted light energy in the on-axis image than either of these prior art elements. The invention also relates to a method of forming this unique HOE.